1. Field of the Invention
This invention relates to a lightweight nonwoven sheet of polyethylene plexifilamentary film-fibril strands. In particular, the invention concerns a particular sheet of this type which is made by a process that includes a specific hot-stretching step.
2. Description of the Prior Art
Nonwoven sheets made from plexifilamentary strands of polyethylene film fibrils are known in the art. Blades et al., U.S. Pat. No. 3,081,519, discloses flash spinning of plexifilamentary strands of polyethylene film fibrils. Steuber, U.S. Pat. No. 3,169,899 discloses depositing such strands onto a moving receiver to form a nonwoven sheet. Methods of assembling strands deposited from a plurality of positions are disclosed by Knee, U.S. Pat. No. 3,402,227. Improved methods for depositing flash-spun plexifilamentary strands and forming them into sheets are disclosed by Pollock et al., U.S. Pat. No. 3,497,918.
The aforementioned methods have proven technically useful and commercially successful in the manufacture of wide nonwoven sheets of polyethylene plexifilamentary film-fibril strands (e.g., Tyvek.RTM. spunbonded olefin, manufactured by E. I. du Pont de Nemours & Co.). However, sheet uniformity problems are encountered in the known manufacturing processes when the weight of the sheet per unit area is decreased by increasing the speed of the moving receiver. The problems are exacerbated when the throughout per flash-spinning position is increased. The sheet becomes more blotchy in appearance and uniformity decreases to the point that the sheet contains pinholes. Sheets with pinholes are inadequate in end-uses, such as sterile packaging, surgical drapes, and the like. Lighter weight sheets are desired but the prior-art methods are inadequate for producing such sheets.
Several methods are known in the art for bonding nonwoven sheets of polyethylene plexifilamentary film-fibril strands. These methods include hot-air bonding on a tenter frame, pressing between heated platens, bonding while restrained against a hot roll by a heavy blanket, calendering with hot rolls and point-bonding with embossed rolls. Several methods also have been disclosed for imparting stretch to such nonwoven sheets, as for example in the aforementioned Steuber patent at column 5 lines 37-67, in Reitz, U.S. Pat. No. 3,408,709 at column 2, lines 57-72 and in Reitz, U.S. Pat. No. 3,406,033 at column 5, line 64 through column 6, line 71. In addition, other general processes for stretching webs are disclosed, for example, by Nash, U.S. Pat. No. 3,208,100, Vogt, U.S. Pat. No. 3,772,417 and Akiyama et al., U.S. Pat. No. 4,187,343. Each of these stretching processes, however, when applied to wide nonwoven sheets of polyethylene plexifilamentary film-fibril strands, has certain shortcomings, such as nonuniformly and excessively shrinking the sheet width and adversely affecting various strength and barrier properties of the sheet.
The purpose of the present invention is to overcome the above-recited shortcomings of known processes and to provide a sheet which can be made at lower unit weight than can be made by known methods and can still possess a satisfactory balance of barrier and properties.